Recently, liquid crystal displays have come into the limelight as one of the most competitive display devices in place of cathode ray tubes in view of their various merits such as light weight and low power consumption. In particular, since a thin film transistor liquid crystal display (TFT-LCD) driven by thin film transistors (TFTs) can independently drive respective pixels, it ensures a rapid response time of liquid crystals, which enables high-definition video pictures. Accordingly, the TFT-LCD has increasingly expanded to its application fields of notebook computers, wall-mounted televisions, etc.
Generally, since linear polarizers are disposed outside both glass substrates in the fabrication of color TFT-LCDs, the color TFT-LCDs are used to display information by adjusting the amount of light transmitted from the rear side through the actions of liquid crystal molecules between the glass substrates. Then, a polarizing degree is adjusted by the amount of light transmitted from the rear side using the liquid crystals, and colors are then generated while the light is passed through a color filter layer, and brightness is adjusted while the light is passed through a linear polarizer, thereby displaying information.
Color filters disposed on an upper glass plate may be divided into dye-type color filters and pigment-type color filters, depending on the materials of used organic filters, and they may be dividedly manufactured according to their manufacturing methods including a staining method, pigment dispersion, electrodeposition, a printing method, etc.
The printing method is used to prepare a color filter by printing red, green and blue inks onto a plate using a variety of printing methods. The representative printing method includes screen printing and offset printing methods, and has advantages that mass-production is possible due to the simple manufacturing process, and it is easy to selectively use materials having high heat resistance or light fastness. However, it has disadvantages that pinholes may be formed by bubbles, or colors may be spread and fast, and squareness in a pattern edge may be easily deteriorated due to the efflux of ink during the printing process.
The electrodeposition method is a method where coloring matters are extracted onto an electrode to form a coloring matter layer, that is, by dissolving or dispersing a polymer resin and a color pigment in an electrolyte solvent to extract pigment into surfaces of transparent electrodes that are in contact with the electrolyte solvent and drying the extracted pigment at high temperature.
For the electrodeposition method, an electrospreading phenomenon occurs when the transparent electrodes have a high specific resistance. Also, the transparent electrodes may be damaged since components present in the electrolyte solvent chemically react with the transparent electrodes. In particular, since the transparent electrodes have relatively low chemical resistance and unusual sensitiveness to damage, their color saturation and light transmittance are deteriorated after the electrodeposition.
Recently, color filters have been mainly prepared by a pigment dispersion method in the case of the liquid crystal display device using a thin film transistor. A composition for forming a color filter layer, which is applied onto a substrate in the pigment dispersion method, includes a polymerizable light-sensitive composition comprising a light polymerization initiator, a monomer, a binder, etc. and a monomeric organic pigment as major components, the monomeric organic pigment being used to display its colors.
The pigment dispersion method is a method where a color filter is prepared from a light-sensitive coloring composition using a photolithography process, the coloring composition being prepared by dispersing a pigment in various light-sensitive compositions. In this case, the use of pigments is desirable in the aspect of stability to light or heat. Also, the pigment dispersion method has been widely used as the suitable method to prepare a color filter, which is then used for a high-precision, high-positioning precision and large color display device, since a color filter is patterned using the photolithography process.
When a color filter is prepared using the pigment dispersion method, a color filter may be obtained by coating a glass plate with a light-sensitive composition using a spin water or a roll water, drying the composition to form a thin film, patterning, exposing and developing the thin film to form colored pixels. In this case, the color filters may be prepared by repeating the above operations with compositions including other colors. However, the conventional pigment dispersion systems have problems that contrast ratio of a color filter is deteriorated and its color variation is induced due to its low polarizing degree by the large size of the pigment particles, and it is also difficult to further improve resolution of a color filter.
In order to solve the above problems, there have been proposed techniques using a dye instead of a pigment. However, the curable compositions containing dyes have problems that their various performances, for example, light fastness, heat resistance, solubility, application uniformity and the like, are inferior to those of the curable compositions using pigments.